Digital Coding of High Quality TV

The study and development of systems for higher quality television has seen tremendous growth in recent years. This has been motivated by the fact that existing television systems give inadequate picture quality, especially for viewing on large screen displays. The main impairments which have been identified an~ scanning artifacts (line stucture visibility and flicker), limited resolution, and luminance/chrominance cross effects. The approaches which have been proposed to reduce these impairments are high-definition television (HDTV) and various enhanced quality television (EQTV) schemes. HDTV involves scanning the original scene with an increased number of lines per frame (usually more than 1000), and with cor­ respondingly greater horizontal bandwidth. This signal is directly transmitted and displayed on a HDTV monitor. EQTV systems have been proposed as a possible alternative to or as an intermediate step towards HDTV. In EQTV the transmission is compatible with the NTSC (PAL or SECAM) standard, at least in scan format, but the quality is improved by signal processing in the transmitter and receiver to reduce the visibility of scanning artifacts and luminance/chrominance cross effects. In this paper, we are concerned with techniques to reduce the visibilty of scanning artifacts. In this form of EQTV system, HDTV cameras and monitors are used. The signal is down-converted to the NTSC scanning standard in the transmitter, and up-converted in the receiver for the HDTV display. EQTV offers a picture quality that is somewhat inferior to HDTV; however, it gives a dramatic improvement on standard N'TSC while maintaining full transmission compatibility. Television signals can be distributed in either analog or digital form. Of course analog transmission is predominant at this time, and the existence of large numbers of NTSC chan­ nels and receivers is the main motivation behind EQTV. If analog transmission of HDTV is foreseen, the analog bandwidth will be at least four times that of the NTSC channel, and generally more depending on the aspect ratio and color coding format used. Digital transmis­ sion facilities are becoming increasingly available and will largely replace analog transmission (except perhaps in over-the-air broadcasting). Digital transmission allows for the use of so­ phisticated digital source coding techniques to make efficient use of the available channel. With such coding, the HDTV signal does not require the large increase in digital transmission capacity that is indicated by the analog bandwidth [1]. This is due to several factors: